The present invention relates to nitride semiconductor devices made of Group III-V nitride semiconductor containing gallium nitride (GaN) as a main component and capable of operating at positive voltages (normally-OFF operation).
In recent years, field effect transistors (hereinafter, referred to as FETs) using GaN-based compound semiconductor materials have been intensively studied as radio-frequency (RF) high-power devices. Nitride semiconductor materials such as GaN enable formation of various mixed crystals together with aluminum nitride (AlN) or indium nitride (InN), and thus are capable of forming heterojunctions in the same manner as conventionally-used arsenic-based semiconductor materials containing gallium arsenide (GaAs) as a main component. However, in a heterojunction in nitride semiconductor, high-concentration carriers are generated at the junction by spontaneous polarization or piezoelectric polarization of the nitride semiconductor even in an undoped state. Accordingly, the resultant FET is likely to be of a depletion type (normally-ON type) and it is difficult to obtain characteristics of an enhancement type (normally-ON type). Devices which are currently distributed in the power electronics market are of the normally-OFF type and such normally-OFF type devices are also strongly demanded for GaN-based nitride semiconductor devices.
Hereinafter, a conventional FET using a nitride semiconductor material will be described.
As illustrated in FIG. 7, a conventional FET using a heterojunction between AlGaN and GaN includes: a sapphire substrate 701 having a principal surface whose surface orientation is a (0001) plane; an undoped GaN layer 702 formed on the principal surface of the substrate 701; and an undoped Al0.25Ga0.75N layer 703 formed on the undoped GaN layer 702.
A source electrode 704 and a drain electrode 705 made of titanium (Ti) and aluminum (Al) are formed on the undoped Al0.25Ga0.75N layer 703. A gate electrode 706 made of palladium (Pd) is formed between the source electrode 704 and the drain electrode 705. A passivation film 707 made of silicon nitride (SiN) is formed on portions of the upper surface of the undoped Al0.25Ga0.75N layer 703 where the source electrode 704, the drain electrode 705 and the gate electrode 706 are not formed.
A two-dimensional electron gas layer at a concentration of approximately 1×1013 cm−2 is formed in the heterojunction between the undoped GaN layer 702 and the undoped Al0.25Ga0.75N layer 703 by spontaneous polarization and piezoelectric polarization of the undoped Al0.25Ga0.75N layer 703.
This formation is explained from distributions of fixed charge and free electrons generated by polarization in the conventional FET, as shown in FIG. 8. Specifically, negative fixed charge is generated in each of the upper surfaces of the undoped GaN layer 702 and the undoped Al0.25Ga0.75N layer 703, whereas positive fixed charge is generated in each of the lower surfaces thereof. In FIG. 8, fixed charge generated in the undoped Al0.25Ga0.75N layer 703 is indicated by the solid lines and fixed charge generated in the undoped GaN layer 702 is indicated by the broken lines. As shown in FIG. 8, the amount of fixed charge generated in the undoped Al0.25Ga0.75N layer 703 is larger than that of fixed charge generated in the undoped GaN layer 702, so that sheet carriers in an amount enough to compensate for the difference in fixed charge amount is formed in the form of a two-dimensional electron gas layer in the heterojunction (indicated by the bold line Ns in FIG. 8).
This polarization causes an electric field in the undoped GaN layer 702 and the undoped Al0.25Ga0.75N layer 703, so that the energy band of the conventional FET is in the state shown in FIG. 9. Accordingly, as shown in FIG. 10, electrical characteristics of a gate voltage and drain current basically exhibit normally-ON characteristics.
The source electrode 704 and the drain electrode 705 are in contact with the undoped Al0.25Ga0.75N layer 703. However, if the thickness of the undoped Al0.25Ga0.75N layer 703 is small, e.g., 30 nm or less, the source electrode 704 and the drain electrode 705 are allowed to be connected to a high-concentration two-dimensional electron gas layer formed in the heterojunction by tunnel current and thus have excellent Ohmic contacts. The gate electrode 706 has a high work function of 5.1 eV, and has an excellent Schottky junction with the undoped Al0.25Ga0.75N layer 703 (see M. Hikita et al., Technical Digest of 2004 International Electron Devices Meeting (2004) p. 803).
In this manner, to achieve normally-OFF operation characteristics using a GaN-based compound semiconductor material which is likely to be of a normally-ON type, it is necessary to reduce the amount of carriers generated in channel by spontaneous polarization and piezoelectric polarization of a GaN-based nitride semiconductor material. In the case of a conventional FET using a heterojunction between an AlGaN layer and a GaN layer, if the Al content in the AlGaN layer is reduced, stress due to the lattice constant difference between the AlGaN layer and the GaN layer is reduced, so that piezoelectric polarization decreases and, as a result, the sheet carrier concentration is allowed to be reduced (see O. Ambacher et al., J. Appl. Phys. Vol. 85 (1999) p. 3222).
Specifically, when the Al content is reduced from 0.25 to 0.15 with the thickness of the undoped Al0.25Ga0.75N layer 703 in FIG. 7 kept at 30 nm, the sheet carrier concentration is greatly reduced from 1.2×1013 cm−2 to 5×1012 cm−2. However, not only the reduction of the sheet carrier concentration reduces operation current, but also the reduction of the Al content of the undoped Al0.25Ga0.75N layer 703 reduces the potential barrier in the gate. In addition, since occurrence of leakage current at the gate electrode needs to be suppressed, the upper limit is set for a forward voltage applicable to the gate electrode 706.
Accordingly, as a method for allowing application of a higher forward voltage in a normally-OFF type device, the gate is formed as a p-type region so as to enhance the potential barrier. FETs having such characteristics are junction field effect transistors (hereinafter, referred to as JFETs) (see L. Zhang et al., IEEE Transactions on Electron Devices, vol. 47, no. 3, pp. 507-511, 2000 and Japanese Unexamined Patent Publication No. 2004-273486).
However, if a JFET is of a normally-OFF type, it is difficult to previously increase the concentration of electrons generated in channel even by applying a forward bias to the gate electrode. In addition, a forward bias is allowed to be applied to the gate electrode until current starts flowing from the gate, and more specifically, the limit is about 3V in consideration of a bandgap. Therefore, a conventional JFET has the problem of difficulty in increasing drain operation current.
FIG. 11 is a cross-sectional view of a nitride semiconductor device as a comparative example for solving the problem described above.
As illustrated in FIG. 11, the nitride semiconductor device as a comparative example for solving conventional problems includes: a sapphire substrate 901 having a principal surface whose surface orientation is a (0001) plane; an AlN buffer layer 902 formed on the principal surface of the substrate 901; an undoped GaN layer 903 formed on the AlN buffer layer 902; an undoped AlGaN layer 904 formed on the undoped GaN layer 903; a p-type control layer 905 formed on a portion of the undoped AlGaN layer 904 and made of AlGaN containing a p-type impurity; and a p-type contact layer 906 formed on the p-type control layer 905 and made of GaN containing a p-type impurity at a concentration higher than that of the p-type control layer 905.
The nitride semiconductor device further includes: a gate electrode 907 formed on the p-type contact layer 906 to be in Ohmic contact with the p-type contact layer 906; a source electrode 908 and a drain electrode 909 formed on the undoped AlGaN layer 904 to sandwich the p-type contact layer 906 therebetween; and a passivation film 910 formed on the region of the upper surface of the undoped AlGaN layer 904 except for regions where the gate electrode 907, the source electrode 908 and the drain electrode 909 are formed.
In this nitride semiconductor device, the bandgap of the undoped AlGaN layer 904 is wider than that of the undoped GaN layer 903 serving as a channel region. The undoped AlGaN layer 904 and the p-type control layer 905 are made of an identical material, so that the bandgap of the p-type control layer 905 forming a p-type control region is also wider than that of the undoped GaN layer 903. In addition, a heterobarrier is formed at the interface between the undoped GaN layer 903 and the undoped AlGaN layer 904, and a two-dimensional electron gas layer is formed during operation of the semiconductor device.
In this manner, the p-type control region (i.e., the p-type control layer 905) having a wide bandgap is provided above the channel region (i.e., the undoped GaN layer 903) of nitride semiconductor and the p-type control region is forward biased with respect to the channel region, so that holes are injected into the channel region. This causes electrons in the same amount as the holes to be generated in the channel region, thus obtaining large operation current. With this structure, holes injected into the channel region facilitate generation of electrons in the channel region, thus making it possible to greatly increase the amount of current flowing in the channel region.
In addition, the mobility of holes in nitride semiconductor is much smaller than that of electrons, so that holes injected into the channel region of nitride semiconductor hardly serves as a single unit for causing current to flow. Accordingly, holes injected from the p-type control region are capable of generating the same amount of electrons in the channel region and increasing operating current. As the mobility of holes injected into the channel region approaches zero, the holes serve as donor ions, so that modulation of carrier concentration is enabled in the channel region. As a result, a normally-OFF nitride semiconductor device having large operation current is implemented.
This structure is similar to that of JFETs, but is completely different from the operation principle of a JFET in which the carrier concentration in the channel region is modulated at the junction of the gate electrode because carrier implantation is intentionally performed.
The nitride semiconductor device structure of the comparative example described above has a problem in which when holes are injected from the p-type control region into the channel region, the holes cannot remain in the channel region and flow downward so that the amount of holes which effectively function in the channel region decreases. In addition, the switching speed depends on the recombination speed between holes and electrons, so that the switching speed needs to be increased to increase operation current.